The present invention relates to an RF power module with a hetero junction bipolar transistor used for power amplification of radio frequency and, more particularly, to a technique useful for compensating properties of an RF power module depending on a property variation in a hetero junction bipolar transistor.
Hitherto, in a hetero junction bipolar transistor (hereinbelow, called HBT), a hetero junction is used for an emitter-base junction of a bipolar transistor, and a band gap of the emitter is made larger than that of a base region. As a result, in the HBT, the amount of minority carriers injected from the base to the emitter is small and, while maintaining the emitter injection efficiency high, the base impurity concentration can be made high. Therefore, low internal base resistance can be achieved, and cut-off frequency fT can be improved largely as compared with that in the conventional bipolar transistor.
In M. E. Kim et al., “GaAs Heterojunction Bipolar Transistor Device and IC Technology for High-Performance Analog and Microwave Applications”, IEEE Transactions on Microwave Theory and Techniques, Vol. 37, No. 9, September 1989, pp. 1286-1303, the reliability of an HBT with a compound semiconductor hetero junction of GaAs/AlGaAs is discussed. In particular, it is reported that deterioration in device parameters such as current amplification factor of an HBT suggests deterioration related to a base-emitter space charge region which may be contamination, surface leak related to ohmic metal, or device short-circuit.
Since the current amplification factor β of a bipolar transistor made of silicon has positive temperature dependency, the grounded emitter current amplification factor increases with temperature rise. It is also written in the document that the grounded emitter current amplification factor of an HBT has negative temperature dependency, and the grounded emitter current amplification factor decreases with temperature rise.